1. Field of the Invention
The present invention relates to a power-operated chuck.
2. Description of the Prior Art
Power-operated chucks of this kind are disclosed in EP 0069917 and EP 0085782. These power-operated chucks have been developed by the patent Assignee and have been successfully marketed by the same for decades. These power-operated chucks have proven effective in practice.
EP 0085782, in particular, shows that each of the clamping jaws is provided with a compensation clamping jaw set suitable for clamping unevenly shaped workpieces and is adapted to be advanced radially independently from the clamping jaw set. Both the clamping jaw set and the centering clamping jaws are adapted to be actuated by wedge rods.
The chucks developed and sold by the patent Assignee have one technical deficit. This is to say, that if it is necessary to secure workpieces reliably that have a large inherent weight and an exterior diameter of more than 0.5 meters, then chucks of the prior art cannot be used, because workpieces of this kind cannot be centered exactly. Workpieces of this type, for example, are to be used as rotors in large electric motors. These rotors and other rotationally symmetrical workpieces must be manufactured as precision components. Therefore, even during the extensive metal-cutting machining of workpieces of this kind, it is necessary for the workpieces to be clamped as coaxially, or centrally, as possible, in relation to the chuck. This means the longitudinal axis of the workpieces to be clamped must be aligned flush with the longitudinal axis of the chuck.
The chucks of the prior art do make it possible with small workpieces for the existing play between, on the one hand, the actuating element configured as a piston which is mounted in an axially movable arrangement in the base body of the chuck and, on the other hand, the base body. However, the greater the outside circumference of the actuating element, the greater is the amount of play existing between the actuating element, in particular between a piston, and the inside diameter of a base body. Error tolerances of this kind cannot be excluded for reasons of manufacturing requirements.
These bearing play problems cannot be compensated for as inaccuracies in the clamping of the workpiece, rather the inaccuracies are transmitted to the clamping situation of the workpiece. Therefore, there is an inaccuracy between the position of the workpiece and the center of the chuck. It is not possible to compensate for the existing inaccuracies in the subsequent machining of the workpieces, with the effect that there are error tolerances in the final status of the machined workpiece, although such should be avoided in high-precision components.
It has proven to be a further disadvantage in the prior art chucks that clamping errors arise during the machining process which is often time consuming and complicated, because the machined workpiece becomes lighter during the machining process, as a result of having material removed from it. Such working procedures, involving material removal, then result in the original position of the workpiece being changed in relation to the chuck and, therefore, in relation to the machine tool.
The clamped workpieces are rotated by the machine tool in order to be machined, therefore in the case of a chuck with three clamping jaws offset at an angle of 120° in relation to one another, and in particular with exceedingly heavy workpieces of 20 tons inherent weight, for example, it has been observed that one of the chucks which is located from time to time in a certain angular position of the chuck has to carry the weight of the workpiece exclusively, with the effect that chucks of this kind must be configured to be extremely sturdy or with appropriate dimensions, because the individual clamping jaw has to absorb the clamping forces that arise due to the weight of the workpiece.
The purpose of the present invention is, therefore, to configure a power-operated chuck of the aforementioned kind in such a way that, firstly, the workpiece to be clamped is precisely centred in relation to the longitudinal axis of the chuck and, secondly, there is a centering adjustment possibility available permanently throughout the machining process. Furthermore, the chuck in accordance with the present invention provides a reliable and sustained support, even of extremely heavy workpieces, with an inside or outside diameter of at least 0.5 meters. Moreover, it is possible to reduce the number of chucks to a minimum, preferably to two or three chucks.